1. Field of the Invention
The present invention relates to methods for doping semiconductor devices, and, in particular to methods for doping semiconductor thyristors.
2. Description of the Prior Art
The present process flow for making an asymmetric gallium thyristor requires at least two separate diffusion furnace operations. A closed-tube gallium deposition is followed by a spin-on boron and oxidation-drive step.
In the closed-tube step, the first step, a quantity of elemental gallium is enclosed in a tube along with a wafer of n-type semiconductor material and the tube is sealed. The tube is sealed so that the release of gallium vapor from the elemental gallium upon heating can be controlled. That is, the quantity of elemental gallium enclosed in the sealed tube is determined such that the quantity of vapor produced when the element gallium is heated is controlled, typically, to just fill the sealed tube. The sealed tube is then broken open and the doped wafer is removed. A high-concentration n.sup.+ -type layer is created by spinning on boron and driving it in by heating, ordinarily at about 1250.degree. C.